Thunderstone Search Appliance Manual

Allow walk to fetch pages from any host in the specified
domain(s). Any URL (of any protocol) with a hostname ending in any of
the specified domains will be accepted.

E.g. given Base URLs of http://www.example.com/ and Extra Domainsothersite.com,
the Search Appliance will walk all of www.example.com, as well as any URLs
referring to any machine in othersite.com or its sub-domains
(e.g. docs.othersite.com).

This option is not a "restricter" but an "enabler". All hosts
specified will be walked and any others that match the given domain(s)
will also be walked.

Note: This option does not direct the walk to web servers
in the specified domains, like putting them in Base URLs would.
It simply allows walking them - if a reference to them is
encountered via walking the existing Base URLs etc. Thus if no
links to Extra Domains are encountered, none will be walked.